The human brain had been increasing in size for most of the past two million years, and then for the last twenty thousand years it has been shrinking.

We have lost the brain size equivalent of the volume of a tennis ball over that time span.

What might account for this?

The explanation I find most compelling is that as social cooperation has expanded, the number of things we can depend upon others to do has as well.

Therefore, we are able to offload some of the thinking which we previously needed to engage in. It is a way of economizing energy.

Remember, the brain, like AI requires an enormous amount of energy to support.

Our brains constitute only 2% of our body mass, yet they consume 20 to 25% of the energy we require.